In semiconductor fabrication, thin, defined layers of metal are used to make connections between thousands or millions of components such as transistors. In some processes the metal film layers chosen can be difficult to define by chemical etch (i.e., wet chemicals) or dry etch (i.e., etch gases in a plasma) processes. These metal films are typically less than one micron in thickness and a known process commonly referred to as “Lift-Off” has been used to overcome problems associated with standard wet or dry etch processes.
As one example, and referring now to FIG. 1, after a photoresist layer 102 has been deposited on the semiconductor substrate (e.g., an oxide) 101, shown as Step (A) in the figure, a typical Lift-Off process uses a photolithographic masking process to define the photoresist layer 102, shown as Step (B) in the figure. In such a process, the photoresist is aligned, exposed, and developed using a photo mask. Next, shown as step (C) in the figure, a thin layer of metal 103 is deposited over the entire surface including the photoresist layer 102. The portions of the metal layer 103 that are deposited on the surface of the oxide 101 are commonly referred to as the primary metal layer and the portions of the metal layer 103 that are deposited on top of the photoresist 102 are commonly referred to as the sacrificial metal layer. After this metal deposition process, the semiconductor wafer is placed into a chemical bath solution to dissolve the photoresist 102 and thereby lift-off or remove the sacrificial metal layer of the metal layer 103. The primary metal layer of the metal layer 103 is preserved and remains on the surface of the oxide 101 for subsequent film layers, shown as step (D) in the figure.
In such a Lift-Off Process (hereinafter referred to as “LOP”) a profile of the photoresist 102 is sometimes adjusted by developing the photoresist more towards the bottom, down next to the substrate 101 surface, thereby creating sloped sidewalls in the photoresist, shown in the figure as sloped sidewalls 102a and 102b. As is known, these sloped sidewalls minimize the metal deposited on the sidewalls of the photoresist during the subsequent metal deposition step.
There are a number of known processing issues that affect this sloped sidewall and efficiency of the LOP during film separation of the primary and sacrificial metal layers during the Lift-Off process. Some of these issues are caused by changes in photoresist sidewall angles due to temperatures variations during metal deposition. Too high of a deposition temperature can soften the photoresist and change the sidewall angle. As is known, photoresist with high thermal stability should be used for LOP. Tool design can also affect the amount of metal deposited on the sidewalls during deposition of the metal layer. Tools (e.g., Electron Beam and Sputtering) that can provide depositions normal (i.e., orthogonal) to the substrate surface, otherwise known as vertical, work best. If too much metal is deposited on the sidewalls of the photoresist, poor edge line definition, bridging metal called “Fences”, and reduced chip yields occur. If the metal deposition on the sidewalls becomes too thick, the sacrificial metal can be very hard or impossible to remove during LOP, which causes other known problems (e.g., portions of metal are left standing upwards from the substrate surface, a condition known as “ears”).
For these and other reasons, there is a need for an improved LOP that avoids these and other issues.